Hopefully it's not a feature of Kodi 19.0-BETA1 as the screenshot filename's structure has been changed a bit but I couldn't find more info / fresh documentation related.
Action taken while TV playing:
kodi-send --action="Screenshot"
or
kodi-send --action="TakeScreenshot"
or invoked by keyboard (CTRL+S or PrtScr)
The screenshots are made in /storage/screenshots/ folder but...
Currently the Screenshot / TakeScreenshot action does not put the active video frame snapshot to .png file, just the Kodi's active GUI menu or a just black page if no menu is active.
Screenshot names are now starting from:
screenshot00000.png
Everything worked as expected (and documented) in LE 9.2.6 official with Kodi 18.9.0 - if the Screenshot was taken, there's a picture frame (combined with active Kodi GUI menu ) saved to .png file.
Screenshot names were starting from:
screenshot000.png