I took a look at the code and the problem code comes from another file 
All perf-o-meters are normally now defined inside the file ~/share/check_mk/web/plugins/metrics/check_mk.py
The perf-o-meter defined there for interfaces don’t know anything about the bandwidth. The old definition posted above had this knowledge. The changed file posted by myself was between 1.5.0p1 and p24 i think.
Inside the metrics/check_mk.py file you can search for “if_in_bps” to find the locations of the definition.
I also found the commit 10674 FIX Interface bandwith: Add missing perf-O-meter for in/out mea… · tribe29/checkmk@7509d1f · GitHub from last month. It added the missing & “wrong” perf-o-meter for all checks with Bits/second. The Bytes/second where already in the wrong format before.