If there are no free subscriptions available, WinCC changes to poll-mode. You can see that 6 tags are polled with a 1s cycle, and 5 with a 5s cycle. Subscriptions are using a little less traffic, but for such low tag numbers the difference isn't big.

Requesting this with subscription would take 2 subscriptions, may be the number of free subscriptions was lower on runtime start and WinCC changed to poll-mode, and does't change it later on.

But from what I see is that the subscription memory is at it's limits, maybe this causes to use the poll mode.

I'd check if the other server uses that much subscriptions, or may be it's a firmware bug where the CPU doesn't free the subscriptions and the memory correctly. With 40 Subscriptions max I guess a rather old CPU. Even my  over 10 years old 1200 with FW 2 supported 50 subscriptions.

If you've got a WinCC just running without any pictures with tags in it, only the tags used in AlarmLogging and TagLogging are requested.