Only a guess, but considering the timing of JP’s Christmas 2019 and Lostbelt 5, my guess would be that LB5 might actually be the last Lostbelt and followed by a Solomon-like Raid event in 2nd half of December.
That would explain why they skipped the Xmas 18 rerun in favor of pulling in Xmas 19, and maybe some SWII servant(s) play a role in LB5 or the finale, so they couldn’t/didn’t want to delay SWII in favor of another new Halloween event, either.
Disclaimer: I pulled this theory out of thin air
Edit: Ignore ^, ofc I forgot to check first if there’s official info on the number of lostbelts…
Anyway, any other ideas?