haddock: ASSERTION FAILED: file rts/Stats.c, line 1413 , with --flavour=perf+ticky_ghc

When I build like this

time ./hadrian/build -o_build_ticky -j binary-dist --flavour=perf+ticky_ghc     

real    52m28.764s                                                                                                                
user    211m32.191s                                                                                                               
sys     10m13.623s                                                        

it's dog slow (with most of the extra time spent in haddock?) and I get lots of

...
haddock: ASSERTION FAILED: file rts/Stats.c, line 1413                                                                            
                                                                                                                                  
haddock: ASSERTION FAILED: file rts/Stats.c, line 1413                                                                            
                                                                                                                                  
| Create directory _build_ticky/stage1/haddock-timing-files                                                                       | Create directory _build_ticky/stage1/haddock-timing-files      
| Run Haddock BuildPackage: _build_ticky/stage1/libraries/libiserv/build/GHCi/Utils.hs (and 1 more) => _build_ticky/docs/html/libr
aries/libiserv/libiserv.haddock                                                                                                   
| Run Haddock BuildPackage: compiler/GHC.hs (and 616 more) => _build_ticky/docs/html/libraries/ghc/ghc.haddock                    
| Create directory _build_ticky/stage1/haddock-timing-files      
| Run Haddock BuildPackage: libraries/Cabal/Cabal-syntax/src/Distribution/Backpack.hs (and 136 more) => _build_ticky/docs/html/lib
raries/Cabal-syntax/Cabal-syntax.haddock                                                                                          
haddock: ASSERTION FAILED: file rts/Stats.c, line 1413
...        

For reference, a normal build of master:

time ./hadrian/build -j binary-dist                                                                                               
                                                                                                                                  
real    25m44.764s                                                                                                                
user    120m18.564s                                                                                                               
sys     8m44.086s      

(the last 5 min of both are spent in xz #20614 but that does not matter here)