kpathsea-warning when using filenames containing $&?

The MWE, test.tex:

documentclass{article}

begin{filecontents*}{A$&1file.tex}
Hello, World!
end{filecontents*}

begin{document}
input{A$&1file.tex}
end{document}

The MWE does not contain calls to write18.

The filename A$&1file.tex is fine with many file-systems.

Nevertheless compiling the MWE delivers the following warning twice (once when processing the filecontents*-environment; once when processing the input-command) to the console but not to the .log-file:

warning: kpathsea: A$&1file.tex: Unrecognized variable construct `$&'.

Why?

Is/where is this behavior documented? Is this a bug?

(Besides these warnings everything seems to compile fine.
The resulting .pdf-file and auxiliary files look as expected by me.)

---

Here is the output to console:

$ pdflatex -shell-escape test.tex
This is pdfTeX, Version 3.14159265-2.6-1.40.19 (TeX Live 2019/dev/Debian) (preloaded format=pdflatex)
 write18 enabled.
entering extended mode
(./test.tex
LaTeX2e <2018-12-01>
(/usr/share/texlive/texmf-dist/tex/latex/base/article.cls
Document Class: article 2018/09/03 v1.4i Standard LaTeX document class
(/usr/share/texlive/texmf-dist/tex/latex/base/size10.clo))warning: kpathsea: A$&1file.tex: Unrecognized variable construct `$&'.


LaTeX Warning: Writing file `./A$&1file.tex'.


(./test.aux)warning: kpathsea: A$&1file.tex: Unrecognized variable construct `$&'.
warning: kpathsea: A$&1file.tex: Unrecognized variable construct `$&'.
 (./A$&1file.tex) [1{/var/lib/texmf/fonts/map/pdftex/updmap/pdftex.
map}] (./test.aux) )</usr/share/texlive/texmf-dist/fonts/type1/public/amsfonts/
cm/cmr10.pfb>
Output written on test.pdf (1 page, 12015 bytes).
Transcript written on test.log.

---

Here is the .log-file:

This is pdfTeX, Version 3.14159265-2.6-1.40.19 (TeX Live 2019/dev/Debian) (preloaded format=pdflatex 2020.2.13)  21 JUL 2020 00:35
entering extended mode
 write18 enabled.
 %&-line parsing enabled.
**test.tex
(./test.tex
LaTeX2e <2018-12-01>
(/usr/share/texlive/texmf-dist/tex/latex/base/article.cls
Document Class: article 2018/09/03 v1.4i Standard LaTeX document class
(/usr/share/texlive/texmf-dist/tex/latex/base/size10.clo
File: size10.clo 2018/09/03 v1.4i Standard LaTeX file (size option)
)
c@part=count80
c@section=count81
c@subsection=count82
c@subsubsection=count83
c@paragraph=count84
c@subparagraph=count85
c@figure=count86
c@table=count87
abovecaptionskip=skip41
belowcaptionskip=skip42
bibindent=dimen102
)

LaTeX Warning: Writing file `./A$&1file.tex'.

openout15 = `A$&1file.tex'.


(./test.aux)
openout1 = `test.aux'.

LaTeX Font Info:    Checking defaults for OML/cmm/m/it on input line 7.
LaTeX Font Info:    ... okay on input line 7.
LaTeX Font Info:    Checking defaults for T1/cmr/m/n on input line 7.
LaTeX Font Info:    ... okay on input line 7.
LaTeX Font Info:    Checking defaults for OT1/cmr/m/n on input line 7.
LaTeX Font Info:    ... okay on input line 7.
LaTeX Font Info:    Checking defaults for OMS/cmsy/m/n on input line 7.
LaTeX Font Info:    ... okay on input line 7.
LaTeX Font Info:    Checking defaults for OMX/cmex/m/n on input line 7.
LaTeX Font Info:    ... okay on input line 7.
LaTeX Font Info:    Checking defaults for U/cmr/m/n on input line 7.
LaTeX Font Info:    ... okay on input line 7.
 (./A$&1file.tex) [1

{/var/lib/texmf/fonts/map/pdftex/updmap/pdftex.map}] (./test.aux) ) 
Here is how much of TeX's memory you used:
 204 strings out of 494412
 2170 string characters out of 6172415
 55258 words of memory out of 5000000
 3979 multiletter control sequences out of 15000+600000
 3640 words of font info for 14 fonts, out of 8000000 for 9000
 59 hyphenation exceptions out of 8191
 23i,4n,17p,112b,314s stack positions out of 5000i,500n,10000p,200000b,80000s
</usr/share/
texlive/texmf-dist/fonts/type1/public/amsfonts/cm/cmr10.pfb>
Output written on test.pdf (1 page, 12015 bytes).
PDF statistics:
 12 PDF objects out of 1000 (max. 8388607)
 7 compressed objects within 1 object stream
 0 named destinations out of 1000 (max. 500000)
 1 words of extra memory for PDF output out of 10000 (max. 10000000)

message{in   }
immediateopenin5="A$&1file.tex"

message{out   }

immediateopenout5="A$&1file.tex"
immediatewrite5{zzz}
immediatecloseout5
bye

 in warning: kpathsea: A$&1file.tex: Unrecognized variable construct `$&'.
 out  )