Honestly, HLG can get quite complex if you drill down into the details. I personally would not recommend it as a camera encoding if using an ACES pipeline.
As @sdyer said in his earlier post, there is not support for HLG input in ACES. There is a convenience Output Transform which is really an ST.2084 (PQ) output transform followed by an HLG conversion such that on the same 1000 nit monitor PQ and HLG will give identical results. HLG is designed so that picture rendering is applied in the monitor, whereas ACES applies picture rendering in the form of the RRT during conversion between scene and display-referred encoding. They are two very different approaches, and a full explanation is too much for a forum post.
I believe that in RCM, “Rec.2100 HLG (Scene)” uses the formula from ARIB STD-B67 (flip between “ARIB STD-B67” and “Rec.2100 HLG (Scene)” and you will see no change). With ARIB STD-B67 an HLG value of 1.0 encodes a scene-linear value of 12.0, whereas according to ITU-R BT.2100 an HLG value of 1.0 encodes a scene linear value of 1.0. The are the same curve, but with a scale factor of 12 applied in linear.
According to ITU-R BT.2408, the recommended exposure for HLG puts 18% grey at 38%, which fulfils the HLG aim of “looking reasonable” if viewed unmodified on a Rec. 709 monitor, but in doing so only leaves about 4.4 stops of highlight latitude between 38% and 100%. S-Log2 and S-Log3 encode significantly more highlight latitude than that, so I would recommend one of those if using ACES. Even S-Log1has more highlight latitude than HLG exposed like this.
The inverse HLG 1000 nit Output Transform maps 38% to an ACES value of about 0.276 which while higher than the nominal 0.18 for ACES mid grey, is only 0.6 of a stop over so not unreasonable. ARIB STD-B67 on the other hand maps 38% HLG to 0.5776 linear, which is about 1.7 stops above 0.18. Hence the far brighter result you see.
I’ll stop now, as you are probably far more confused than you were originally!
My main recommendation is don’t record HLG if you are using ACES.