It's a pity that you didn't migrate the tickets from the old system. I hope you have a backup somewhere. Under this topic we had an interesting discussion that was still open and waiting for resolution.
Summing it up, Riseup's XMPP is showing some age and needs an upgrade. It misses several XEPs that are a must nowadays. For instance Message Carbons (XEP-0280) and Message Archive Management (XEP-0313), that provide inter-client message and history
syncronization. Check the full list at: https://gultsch.de/compliance_ranked.html
As it stands Riseup's XMPP server is only fit for the old model where each user had a single device and only took part in synchronous chats. Nowadays people expect Instant Messaging to be asynchronous and synchronized across all their devices.
XMPP clients like Conversations implement these new use cases and turned XMPP back into a viable option for the mainstream. It is bringing back people that had previously migrated to more centralized and even proprietary protocols because of the lack of features. However we now need server-side support across the ecosystem to keep the momentum going.