AArch64 drreg failure in drcachesim: "failed to restore reg" on stolen reg value

Running drcachesim on AArch64 with a thread filter hits a drreg failure: "failed to restore reg" on the stolen reg value. Looks like it calls drreg_get_app_value() on the stolen register. Probably it should just skip for that reg since it won't be a scratch reg?

Why didn't the test suite hit this? Looks like the tool.drcacheoff.burst_threadfilter test is disabled (along with many similar tests) on AArch64 due to #2007 (closed) :(