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@oblomov And for completeness, the answer the other question. I _was_ using Monaco in my editor (Emacs) but for the partial blocks it was falling back to Andale-Mono.
Going back to the default which gets me Menlo for partial blocks and full block, and consistent heights.
Adding that to the long list of things to consider when picking my next (coding) font. 
@oblomov That screenshot that I think you're referring to is actually the Discord app in the Firefox browser.
Plot twist!
You were on to something. While writing my response I noticed that I had a bug in my code and was unintentionally using the "left 7/8 block” U+2589 where I thought I was using the full block.
Back to the drawing board (hah) for me, and more 
️ feelings with regards to fonts and unicode. 
@oblomov I like those sparklines
As far as I can tell (checked again just now), I'm using these: 2588 for the full block, and 2589..258F for the (left) 1/8..7/8 partials.
I want to use the unicode (partial) block symbols (https://strangeweb.page/@jochie/115118219770375106) to draw graphs:
chars = [' ','▏','▎','▍','▌','▋','▊','▉','█']
The idea is that I can "scale” the horizontal bars (7/8, 6/8, 5/8, …) if/when at least one of the values gets a bit high, like this:
1️⃣ 0
2️⃣ 0
3️⃣ ▉▉▉▉▉▉▉▌ 12
4️⃣ ▉▉▉▉▉▉▉▉▏ 13
5️⃣ ▉▉ 3
6️⃣ 0
In my editor/client it’s fine, but some fonts show
U+258C “▌” as taller than the others, which is a bit annoying!
Any suggestions/alternatives?