Can It be an area with a wind shift and an updraft?
The first would explain the GS increase, the second would explain the IAS/TAS increase, (if level flight and power were maintained during the occurrence).
In addition, if pitch had to be lowered to maintain altitude, an updraft could be considered, as this would mean the IAS increment was actual, not just a wrong indication.
There would be propeller effects, too.
Expert glider pilots can "feel" where the gusts come from, and of course the updrafts and downdrafts.
A gust is transient, but an updraft can be sustained. Normally a head wind decrease will produce a decrease in IAS/TAS followed by an increase in GS until all is back to pre-gust conditions. But a simjultaneous updraft can mask the decrease in IAS or even overcome it and turn it into an IAS/TAS increase.
Of course I am just an amateur who has never been even close to a test pilot school, sorry if bothering you guys.