This was hurting my brain for a while and I have a solution at least for my setup.
Setup - Hifiberry Digi+ Pro with RPi 4 something like 1.4, it’s before the 1.5 that HFB say is problematic.
Symptom - power system off. Then power it on. Now ALSA troubles on pressing play.
Fix - 1. Power up and wait for system to be FULLY loaded again (note this takes a while even after webUI is accessible)
2. then go to the audio output, switch off I2S DAC and select Headphones (HDMI might work too not tried)
3. SAVE!!
4. Reboot
5. Play something even for a second. Just let the track location start progressing. This seems unnecessary but appears to help.
6. STOP the playback. Essential or it’s gonna still be screwed. From memory.
7. Back to Audio Output. I2S DAC ON again.
8. Select Digi+ Pro - there’s no Digi2 Pro as further evidence that God no longer watches over us 
9. SAVE
10. Reboot.
11. A minor miracle - the thing starts to play correctly through the Digi2 Pro. Evidence that God had half an eye on and is an audiophile. 
It shouldn’t be this awkward… but at least it works I suppose. Got me pretty tempted to find another hat with better quality componentry - wouldn’t have expected incompatibility with v1.5 and partial incompatibility with v1.4.
Peace out 