I have exactly the same issue. It only started for me with the 3.6.0 update but I also updated some INDI drivers around the same time. I'm also using a ZWO EFW 7 filter wheel.

Have you found a work around for this issue yet? It's a bit of a pain updating the FITS header one image at a time.

The problem went away when I installed astroberry on a clean micro-sd. The original install had kstars 3.5.7 or 3.5.8, but I could update to 3.6.0. No problems whatsoever.

The solution I’ve been using successfully until EKOS gets fixed is that I modify the file name rather than the FITS header. My file names from EKOS always include the filter name. I use PixInsight for my processing and if I insert the word “Filter_” before the filter name in the file name, PI is able sort by filter. I’m a MacOS user and there’s a simple rename utility in the OS that I use.

I have tried using PI to change the FITS header but it’s been a bit of a nightmare. Using file and process containers often causes the entire FITS header to be re-written. Some people have used PI scripts as well. Using the file name change has been much easier for me and I can change 30 files in 5 seconds.

I also include the filter name in the filename but is PI's WBPP script able to use the filename for the filter rather than the header?

Something else I should mention... this started when I upgraded to KStars 3.6.0 but at the same time I updated the "indi-asi" driver from version 2.0 to 2.2. I've noticed looking through my logs that all my problems are with v2.2 of the ASI driver. I wonder if that's the problem rather than Ekos? I have been unable to find a repos to downgrade to v2.0 to test this.

Thanks
Ken

FYI, I just checked the driver version I am using: indi_asi_ccd v 2.2

In my experience PixInsight’s WBPP script picks up the filter when I add the word “Filter_” (with the underline as the separator between the word Filter and the name of the filter) before the filter name. The fallback is that if fails, you can use WBPP Keyword functionality to accomplish the same thing. I’ve not had to fallback to that yet as the addition of the word Filter seems to provide WBPP with enough to properly sort the filters.

I can’t say exactly when EKOS stopped writing the FITS header properly but it is a problem that I hope someone is addressing.

The issue should be fixed in 3.6.1 with optical trains.

Also in my situation filter names were intermittently (not) part of the FITS header. Just installed 3.6.1 and did a few test runs with both light and flat frames, and all filter names were perfectly added to the fits header. Just like Jasem is saying, it seems that this bug is fixed, thank you!
For those who want to manually add the filter name in the fits header, you may want to check out this PixInsight script . It's called BatchFITSKeywordEdit. You can load a whole list of files and add, edit or remove keywords like you wish.
Here is a screenshot: