I think that you are there where you want to be (at the finish line), even if this not for certain (as I’m not some expert). Seriously, what I can see on related picture is that your Librem 5 battery have chosen to be charged with D+ of 0.07V and D− of 0.07V (reaching almost 4.20V, or through another viewpoint reaching its max. capacity, just my distant guess based). My related thought, at this moment, is that D+/D− is more related toward which kind of power supply unit you are using to charge your here related Linux phone and that this resulting handshake (negotiation) comes from your power bank at the first place (best charging option/protocol at 5V selected or even the only one available, telling us that pure PD power supply used there as source).
Just my two cents and as linked earlier, probably ideal charging protocol at 4.50V to 5.20 volt range might be closely related to 0.00V/0.00V for D+/D− (or rather equals to those values toward the end of actual charging cycle, of some Li-ion battery), so I’d consider this type of charging your Librem 5 battery as perfect, IMHO. For example, one of my USB-A based power banks just finished charging my Librem 5 with D+/D− at 0.20V/0.20V (under very same charging protocol as on your picture shown on another, yet quite similar USB-C meter you’ve used here for this measurement).
And as I just found in my archive, this would be demonstration (while D+/D− registered at 0.085V/0.085V) of what I’ve tried to explain within above sentences, under both sides (source<=>sink) negotiated or rather both sides triggered (choice of this word should be considered as correct here) PD2.0 charging protocol (and I hope that this approach, as shown, helps you further building some conclusion of your own):
@guru, while I’m considering below as very precise description, I’d also like to remind to this expert explanation as well: