Adding `Enter` before last symbol in sence/antisense chain doesn't work (e.g. not brake backbones on two chains)

[AlexeyGirin]

Steps to Reproduce

1. Go to Macro - Sequence mode (clean canvas)
2. Load from HELM: RNA1{R(A)P.R(A)}|RNA2{R(U)P.R(U)}$RNA1,RNA2,5:pair-2:pair|RNA1,RNA2,2:pair-5:pair$$$V2.0
   
3. Switch sequence to edit mode (SYNC is ON) and move cursor to the middle of sense chain
   
4. Press Enter key

Actual behavior
System starts new sequence

Expected behavior
System brakes backbones on two chains

ALTERNATIVE CASE: In SYNC mode OFF it doesn't work as well for sense and antisense chains

As per requirement:

2.2. If a line break (enter) is added, both backbone chains should be broken.

Versions

• Ketcher Version 3.2.0-rc.4 Build at 2025-03-25; 16:50:39
• Indigo Version 1.30.0-rc.4.0-g552c98211-wasm32-wasm-clang-19.0.0
• Chrome Version 134.0.6998.178 (Official Build) (64-bit)
• Win10

Issue found while testing: #6254

[SefaBilgii]

Look like this issue not reproducible

Screen.Recording.2025-08-01.at.10.20.24.AM.mov

Ketcher Version: 3.6.0-rc.1
OS: MacOs Sequoia Version 15.5
Browser : Chrome Version 138.0.7204.169