Welcome Bob,
Strange one that!
When you say ‘when the car warms up…’.
Could you try a little experiment ( if not already tried).
Start the car as normal, but do not attempt to operate the flashers.
Do not move the car, but leave it idle until the cabin temperature reaches a sensible level - even though the outside temperature is still low.
Once the cabin is reasonably warm, try the flashers, and report back.
If they still don’t work, then road test the car and see if they start working at less distance covered, than they did when the car was moved from cold.
Kind regards,
Gareth.