CalDav (Baikal) calendar keeps original reoccuring entry when a single occurence is changed

Since there is no longer support for Outlook calendars (given the depreciation of basic auth suport in 365) we have started using Baikal CalDav.

We have a reoccuring entry in our calendar which syncs to freepbx just fine.

When we edit one of the occurences in the series, it does not delete the original entry, it creates a new single entry but the occurence from the series is still there.

Using Baikal Caldav calendar, we check the ics entries from here and it seems that the series has changed the reoccurence as we would expect. Any idea as to why FreePBX shows the series and the new single changed occurence?

Example:

Original series of reoccuring event (reoccuring every 8 weeks):
image

We edit just one occurence (in this example change start date from 9th Jan to 10th Jan):

Upon checking Baikal, the ics download shows the change correclty (only one occurence that week with no entry on the 9th as above)

Upon syncing, FreePBX keeps the original reocurring entry and create a parralel entry for the single occurence change: