Chemfig: submolecules not printing properly after upgrading from TeXLive 2015 to 2020

Here is a MWE of the code I’m talking about:

documentclass[12pt]{article}
usepackage{chemfig}
definesubmol{MP}[-[4]P(-[3]O^ominus)(-[5]O^ominus)=[4]O]{-P(-[1]O^ominus)(-[7]O^ominus)=O}
definesubmol{MA1}[(-[7])-[4](-[5]O^ominus)=[3]O]{(-[3])-(-[1]O^ominus)=[7]O}
definesubmol{MA2}[(-[1])-[4](-[5]O^ominus)=[3]O]{(-[5])-(-[1]O^ominus)=[7]O}
begin{document}
chemfig{--[:300]N(-[1]!{MP})-[:330]-[6]-[:210]N(-[7]!{MP})-[:240]-[4]-[:120]N(-[5]!{MP})-[:150]-[2]-[:30]N(-[3]!{MP})-[:60]}
end{document}

After upgrading to TeXLive 2020 (through MacTeX), the submolecules print out the actual string instead of rendering them as structures (see image).

Before when I had TeXLive 2015, it was printing properly. Does anyone know how to fix this? Thank you!

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EDIT: A related question with a shorter MWE may be found HERE.