Hi @JBorgers,
This might be because of the HubSpot Sales Outlook desktop add-in.
The cursor jumping around (or dropping to the bottom) is a known issue; switching to the HubSpot Sales web add-in has solved this in many cases.
Reference thread: https://community.hubspot.com/t5/Outlook-Sales-Extension/Cursor-jumping-at-the-bottom-of-the-email-chain-when-replying/m-p/1191015
If this helps, feel free to mark it as the solution
and give it an upvote
!