Unexpected tlmgr behaviour

I have the following unexpected behaviour when I run tlmgr

• Running tlmgr --version tells me I’m running TeX Live 2020, namely the output is:

tlmgr revision 57119 (2020-12-13 00:40:00 +0100)
tlmgr using installation: /usr/local/texlive/2020
TeX Live (https://tug.org/texlive) version 2020

• Running sudo tlmgr --gui tells me I’m running TeX Live 2019 (‽), in particular:

tlmgr revision 53568 (2020-01-27 19:20:16 +0100)
tlmgr using installation: /usr/share/texlive
TeX Live (http://tug.org/texlive) version 2019

• Running sudo su and then tlmgr --gui gives me again TeX Live 2020, with output:

tlmgr revision 57119 (2020-12-13 00:40:00 +0100)
tlmgr using installation: /usr/local/texlive/2020
TeX Live (https://tug.org/texlive) version 2020

Why is that, and how do I fix it?

I tried removing /usr/share/texlive hoping that if tlmgr won’t find it it would go about to search for /usr/local/texlive/2020, but no luck.

It should be noted that I installed texlive 2020 on a fresh system, i.e. I never ever installed texlive 2019, which makes this behaviour even more surprising.