I can confirm the reported issue. I didn’t notice it because I use to make Signal calls with BT headset. So I tested this issue in my last call.
With Shiftphone I made the same disappointing experience as with iodé: They don’t feel responsible for LineageOS bugs. The Shiftphone staff member, that maintains the LineageOS ROMs in his free time is busy with working on the OS for their new phone. So maintanance on the SHIFT6mq LineageOS ROM is very unlikely. And there are no reactions on issues created on Github.
OT: The only reason why I don’t buy lifetime iodé Premium is that I don’t know how long it will be usable on my phone without proper maintenance.