pdflatex on TL2020 complains about not finding l3backend-pdfmode.def

[AC]

I have two machines running vanilla-installed TL, migrated[0] from 2019 on both.

I have compiled with pdflatex on both just fine. 

After a recent upgrade, running 'pdflatex' (i.e. the command pdflatex <tex-file> in a linux terminal) on one of them (henceforth BAD) results in the error that l3backend-pdfmode.def cannot be found.

On the other one (henceforth GOOD) everything works fine, even though l3backend-pdfmode.def is not present there either.

On both machines I have l3backend-pdfmode.def in <path-to-texlive>/2019/texmf-dist/tex/latex/l3backend/

but GOOD runs without issues without that file in the analogous 2020 folder, whereas on BAD I have to copy that file to 

<path-to-texlive>/2020/texmf-dist/tex/latex/l3backend/

to fix the problem.

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References:

[0] https://tug.org/texlive/upgrade.html