Safari web-inspector (CSS editor) is extremely bugged

Question

Safari's web-inspector -more specifically the CSS editor- is extremely bugged. In the example below I entered font-size: 0.8em; and hit enter, but nothing changed on the website and the code I just entered disappears. So I decide to enter the same code again, but after I hit enter now this happens:

Here's what I know so far:

• It always happens in the exact same order. First try nothing happens and the code disappears. Second try the code shows up like in the image.
• Most of the time, the list of "bugged code" is just a few rows and I can remove it to try again. On some rare occasions the "bugged code" is 100-200 rows.
• Usually, after 2-5 tries, the code will no longer get bugged and the changes will apply to the website I'm using web-inspector for. It just takes a couple of tries.
• This only happens in Safari. All other browsers I installed, such as Chrome and Firefox, still work without any issues. No bugged web-inspector at all.
• The bug can happen with all CSS properties, so it doesn't have to be font-size. Also margin and basically any other code does this exact same thing.
• Every single website I tried is susceptible to this bug, so it doesn't just happen on one of my own little projects. It's a real Safari bug I believe.
• It only happens to CSS properties. So any other website element I attempt to change, including CSS selectors, do not become bugged at all.
• I'm not 100% certain about this, but it seems to happen only to CSS properties that (partly) require numbers as input, such as 0.9em, 85%, 16px, etc.

Safari is up-to-date, but I have no idea for how long I've been experiencing this issue. It's been at least 6 months now and I've sort of "learned to live with it". But now I'd really like to resolve this issue and I have no idea what could possibly cause this. On Safari I have no extensions or anything installed. Just a plain and simple Safari installation.

Did anybody experience the same issue?

How do I solve this?