Strange UDP behavior when receiving data

Dear all,

I am now struggling with this topic since a while and hoping that someone can help.
I have OH running on a Synology. First 1.8.3-001 and after I wasn’t able to solve this issue I moved to
But I still see the same strange thing happening.
I have built a temperature and humidity sensor based on a particle photon. It can be polled and then sends the temperature and humidity via UDP from two different ports.
Temperature from port 8002 and humidity from 8003.

But now I am struggling in receiving this data in a proper manner.
My item setup looks like this:

String Temperature "Value: [%s]" { udp="<['REGEX((.*))']" }

But this doesn’t work, except when I first configure it like this:

String Temperature "Value: [%s]" { udp="<[*:'REGEX((.*))']" }

and then change it back to the port 8002.

After a reboot it stops working again.
Anyone seen this behavior already?