I was sure this had been posted months ago but a search turned up nothing, so posting now. Mods with lockout duration can normally still gain power from power over time sources like archon prime, cataloger's, and feedback refund. Heatwave cannot. Notably, it CAN still gain power from damage dealt while it's running if leaking gemstone is equipped, just like any other lockout mod, and damage from a second mod equipped with feedback can also give it mod power (note: NOT the 20% charge cost refund, that part doesn't work) - it seems to be just these power over time effects causing problems.
Simplest repro: equip archon prime and cast heatwave. When the duration ends, it will start recovering from 0 power: it should be substantially filled from the prime perk mod generation while it was active.