According to the information I found on the minecraft wiki, Bedrock Edition wither will Regenerate 1 HP/sec when there are no players within 50 blocks, and testing has confirmed that this behavior does occur.
But Spectator mode player will also prevent Regeneration behavior when it is near wither.

Reproduction Steps:

1. Spawn the Wither (recommended to keep it restrained).
2. Deal some damage to it (suggested 1–10 points).
3. Switch to spectator mode.
4. Observe the health bar (boss bar).
5. Move away from it (over 50 blocks).
6. Observe the health bar (boss bar) again

Expected behavior:
Spectator mode player does not affect the behavior of wither

What actually happened:
Spectator mode players prevent wither's Regeneration behavior