The long-standing (since stereo was a new fancy thing) convention for mono output on a multi-speaker system is to send it to FL/FR so that's not unexpected. The HD formats are not mono, they are always multi-channel. So what you describe as "DTS 1.0" is more correctly described as 6-channel DTS where FL/FR/RL/RR/LFE channels have inaudible content and FC channel has audible content. If you disable pass-thru and have set Kodi to downmix to 2.0; that's what Kodi does, it downmixes multiple channels (including the one with audible content) to the default speaker map for 2.0 audio which is FL/FR. So everything working normally and not a bug.
It's probably possible to use alsa confs to define a multi-channel arrangement that remaps true mono to a multi-channel FC-only arrangement but I don't have any links to share and it's a complete bitch to Google for anything alsa related - all you ever get is a bazillion "not quite what you're looking for" results returned. I'll duck the opportunity to think any further about that idea.
My $0.02: Claim the sofa sweet-spot equidistant from FL and FR and then it all sounds like it's in the centre anyway