I just had the same problem with a Viiiiva HR strap; pretty much exactly the same symptoms (1.0 training effect, 0 hours recovery time). I've had multiple successful runs with the strap before, though, so it's not incompatible. E.g., last week I had a very similar run with exactly the same setup (735XT + Viiiiva HR strap + Stryd footpod) and it showed training effect 3.6.
However, I noticed that I lost contact with the HR strap ~20 seconds during the run, so there's some data missing in the middle. (The rest looks perfectly normal.) Could that be a possible reason?