strange behaviour of fontseries

Question

In the following MWE, "Lorem ipsum" should be light, but you get the regular weight.
%RequirePackage[2019/10/01]{latexrelease}
documentclass{article}
  
begin{document}

fontseries{l}fontfamily{NotoSans-TLF}selectfont Lorem ipsum

end{document}

The warning says:
LaTeX Font Warning: Font shape `OT1/cmr/l/n' undefined
(Font)              using `OT1/cmr/m/n' instead on input line 6.

Apparently, LaTeX tries OT1/cmr/l/n (which doesn't exist), substitutes OT1/cmr/m/n, and finally you get OT1/NotoSans-TLF/m/n.
If you use RequirePackage[2019/10/01]{latexrelease} (this cancels the NFSS changes of LaTeX 2020-02-02), you get the light series as expected.
Further observations:

If you ask for fontseries{sb}, you get b instead.
If you ask for fontseries{c}, you get m instead.
If you ask for ul, el, m, b, eb or ub, you get the requested font series. (This is very confusing because e. g. OT1/cmr/ul/n doesn't exist either)
If you use fontfamily{NotoSans-TLF}fontseries{l}selectfont Lorem ipsum instead, you get the requested font series.
The same problem occurs with other fonts.

egreg · Accepted Answer

Font attributes are changed in the order
fontencoding
fontfamily
fontshape
fontseries
fontsize

So what happens with your input is that first the shape is changed to l, which Computer Modern hasn't, so it's substituted with the default m.
Using the proper order the output is as expected:
documentclass{article}

begin{document}

fontfamily{NotoSans-TLF}fontseries{l}selectfont Lorem ipsum

end{document}

Most important is to have the family declaration at the beginning, because this will cause LaTeX to read the font definition file (if not already done) so the possible other attributes are known.

Ulrike Fischer · Answer

The difference between the ul and the l series is that the LaTeX kernel has a rule for the m/l switch:
DeclareFontSeriesChangeRule {m}{l}{l}{m}

This rule means that if m (first argument) is the current series, and l is requested (second argument) then l is tried (third argument) and if it doesn't exist the series falls back to m (fourth argument).
As egreg wrote, you are trying to change the series while cmr is still active, and so, as it has no l-series, m is forced.
You can get a similar effect for ul if you declare a rule for it too:
documentclass{article}

begin{document}

{ fontseries{ul}fontfamily{NotoSans-TLF}selectfont Lorem ipsum }

DeclareFontSeriesChangeRule {m}{ul}{ul}{m}

{ fontseries{ul}fontfamily{NotoSans-TLF}selectfont Lorem ipsum }

end{document}

You can avoid the fallback to m by removing the fourth argument from the rule:
documentclass{article}

DeclareFontSeriesChangeRule {m}{l}{l}{}
begin{document}

fontseries{l}fontfamily{NotoSans-TLF}selectfont Lorem ipsum

end{document}

strange behaviour of fontseries

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