System shows phosphate as CHEM in backbone chain

[AlexeyGirin]

Steps to Reproduce

1. Go to Macro - Flex mode (clean canvas)
2. Load from HELM: RNA1{R(U)P}|RNA2{P}|RNA3{R(A)P}|RNA4{R(A)}$RNA1,RNA2,3:R2-1:R1|RNA3,RNA1,2:pair-2:pair|RNA4,RNA2,2:pair-1:pair$$$V2.0
   
3. Switch to Sequence mode

Actual behavior
System shows phosphate as CHEM in backbone chain

Expected behavior
System shows phosphate as it is

Versions

• Ketcher Version 3.1.0-rc.1 Build at 2025-02-04; 21:27:02
• Indigo Version 1.29.0-rc.1.0-g2e30df8f7-wasm32-wasm-clang-19.0.0
• Chrome Version 133.0.6943.54 (Official Build) (64-bit)
• Win10

Issue found while testing: #5942