I've just stumbled over this after an upgrade to buster, and can provide an additional datum that can also be used as a workaround:
While direct export to PDF via pdflatex is broken, the resulting TeX file is fine. That means that you can export the LyX file to LaTeX (pdflatex), and then from the command line invoke pdflatex on the file to obtain a PDF. (And to no surprise, it also affects german Umlauts). Kind regards chrysn
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