Willem, I noticed this issue first with Flats and 3.5.7. I updated to 3.5.8 three days ago, just before a rare clear night two nights ago. I noticed the issue when PixInsight's WBPP script reported a number of Flat exposures fell into a "No Filter" category. This time a few light frames were effected too. Filenames properly show filter used but FITS header is missing the Filter keyword. Last time I think only Flats were involved.
I'm hoping for a resolution as individually adding the proper keyword and value to each file is time consuming. I haven't the skills/experience to write a script to automate the process.