This is an erratic behaviour which I cannot reproduce when I want. When using writer scrolling with the mouse wheel suddenly stops. However, clicks from the other mouse buttons are picked up and it is possible to scroll using the keyboard at that time. The only way to regain mouse wheel scroll is to click any item on the menu, open the dialogue box and cancel it. For instance, Help > About LibreOffice > Close, or Insert > Image… > Cancel
My installation details are:
Version: 6.4.7.2
Build ID: 1:6.4.7-0ubuntu0.20.04.1
CPU threads: 12; OS: Linux 5.11; UI render: default; VCL: gtk3;
Calc: threaded
The same behaviour existed in 6.4.6 and the recent upgrade to 6.4.7 has not made any difference.