From the linked forum post:
These are my software versions, although this problem has been persisted for a month:
Thunderbird Version: 68.2.1 (32-bit)
Lightning Version: 68.2.0
TbSync Version: 2.7
My weekly routine involves adding descriptions to recurring events. Before migrating to Thunderbird/Lightning (Remote Calendar via CalDAV hosting provider) from Outlook (Remote Calendar via Exchange/Office Enterprise), it was the easiest and quickest way to have detailed recurring events: The meeting itself is recurring, and always at the same location, just every week I add a description. The modified event itself was not part of the recurrence anymore, but that was fine.
Now, in Thunderbird, I run into this problem: If I have a recurrent event (with time and location set) and modify a single occurrence (adding a description), the modified occurrence will have "forgotten" its location. Even worse, while the editing dialog is still open, it will show the location, but when I then save and close it, the location is gone (both in the calendar view, and when I open it again).
The problem is appearing using both Lightning CalDAV as well as TbSync CalDAV.
If I change the description and the location for a single occurrence, it remembers both correctly.
If I use my web-based CalDAV interface from the hosting provider, editing a single occurrence keeps the location, and it also syncs back correctly to Thunderbird.
I have tried the same for local calendars and the problem is not appearing in this case.
Editing a single event keeps the location information correctly in Thunderbird 60.9.1 (32-bit) / Lightning 22.214.171.124.
If I export the local test calendar, which only includes the series as well as the modified event (and appears correctly in Lightning), the .ics file (attached below) does not contain the location information in the modified VEVENT. Importing it (even to another, local test calendar) also does not reload the location from the reoccurring event.