I have this same problem using my RPi as a server at the mount. It saves the Filter keyword on the very first file captured after startup but then none afterwards. Been this way for a while. I am running Stellarmate OS on the RPi and  used Software Updater to update the OS yesterday before my run last night. Not having the Filter designation keyword in the Fits Header is inconvenient when using Weighted Batch Preprocessing Script 2 in PixInsight. All the frames (including Flats) have to be added using the Add Custom button which then requires filling out by hand information about the files. Also my Mac version (3.5.5) of KStars/Ekos does not record the Filter keyword in the Fits Header when capturing files - normally use this set up when recording Flats with my indoor panel. I am using the ZWO ASI6200 MM and EFW filter wheel.

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