Hi Svein,
I can sucessfully repro the problem you're experiencing on TMOS v12.1.1 HF2. So you may open a support ticket to clarify the situation and/or make the F5 support aware of the problem. In the meantime you can use the syntax below to manually parse the HTTP version out of the [HTTP::request].
set http_version [string range [lindex [getfield [HTTP::request] "\n" 1] 2] 5 end]
Additional Note: The [matchclass] and $::your_datagroup syntax is deprecated since v10. I encourage you to migrate your code to become a [class] compliant syntax, to avoid performance degration, unexpected results and/or even TCL runtime errors.
https://clouddocs.f5.com/api/irules/class.html
Cheers, Kai