Depending on how the foreground app is coded, you may be able to get around this.

First, it happens when a foreground app is also playing audio, or doing something invisible to the user with audio. The latter is always tricky, but my understanding is some apps play inaudible tones to fingerprint your device for ad targeting, which will pause already playing music even though the foreground app doesn’t appear to be making any noise.

Anyways. The way around it is instead of switching back to Spotify to resume your audio, instead pull down control centre while you’re still in the app you want to use. From control centre press play. This will often, but not always, work.