ok, finally figured this out.
turned out to be a stream profile on the virtual server running the iRule. in 11.2 it's fine to have 2 regular expressions concatenated without a space. e.g.
@http://somesite\.@https://somesite.@@http://someothersite\.@https://someothersite.@
but in 11.6, this silently breaks and sets content-length to 0
using this works in 11.6:
@http://somesite\.@https://somesite.@ @http://someothersite\.@https://someothersite.@
if someone from f5 is reading this, that syntax error should raise an exception or log something somewhere