When Avast Free Antivirus for Windows is installed, it copies api-ms-* libraries as a side-by-side package to a subdirectory of winsxs; but it also copies ucrtbase.dll to System32 directory. This breaks, e.g., Microsoft’s own VC Redist 2015 or 2017 package installer’s UCRT detection logic, which is checking if ucrtbase is present in the directory (the installers are available at https://support.microsoft.com/en-us/help/2977003/the-latest-supported-visual-c-downloads). In this case, the VC Redist would omit installation of the relevant MSU package, which leads to other applications depending on UCRT later failing because of api-ms-win-crt-string-l1-1-0.dll missing.
A workaround for other applications would be to directly install relevant MSUs (available from https://www.microsoft.com/en-us/download/details.aspx?id=48234). In LibreOffice installer, we have workarounded this by trying to install the MSUs unconditionally (ref: https://bugs.documentfoundation.org/show_bug.cgi?id=119910). But it would be good if Avast would not install parts of UCRT to the place where only properly installed library must reside.