Alright, I think I finally figured it out.
I used to follow the steps below to produce a custom symbol when using 2.0 templates:
Export similar 2.0 template from SF Symbols
Import template in Adobe XD
Edit/create custom symbols in desired scales and weights
Export as .svg
Import in Xcode
Now, it seems that you have to import it back into SF Symbols in-between 4 and 5, you can't bring it directly into Xcode. The steps thus become:
Export similar 3.0 template from SF Symbols
Import template in Adobe XD
Edit/create custom symbols in desired scales and weights
Export as .svg
Import into SF Symbols
(Set up layers and colors for the various rendering modes)
Export symbol
Import in Xcode
maxkve
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In the SF Symbols app, symbols look fine:
But in Xcode, they'll look like this:
Or like this: