Quarto doc: Markdown grid_table destroyed when returning to 'Source' editor

Bug description

System details:
Windows 11 Pro
Intel(R) Core(TM) Ultra 9
RAM 64.0 GB

Positron and OS details:
Positron Version: 2025.09.0 (system setup) build 139
Code - OSS Version: 1.103.0
Commit: https://github.com/posit-dev/positron/commit/30e3d0a74e09a0f6640929822cb57a326671360e
Date: 2025-08-29T18:46:05.223Z
Electron: 37.2.3
Chromium: 138.0.7204.100
Node.js: 22.17.0
V8: 13.8.500258-electron.0
OS: Windows_NT x64 10.0.26100

Session details:
Quarto document
R 4.5.1 (R not used in this doc)

Describe the issue:
Markdown grid_table mostly destroyed when returning to 'Source' editor, after first switching to 'Visual' editor

Steps to reproduce the issue:

• Quarto doc containing two Markdown grid_tables in Source editor mode
• Switch to Visual editor
• return to Source editor
• 1st table has majority of number values deleted; second table almost disappeared entirely.

Expected or desired behavior:
Document survives unscathed

Were there any error messages in the UI, Output panel, or Developer Tools console?
unsure.

Thank you.

Steps to reproduce

No response

Actual behavior

No response

Expected behavior

No response

Your environment

No response

No source file, test, or entry point is named. Start by reproducing the issue with a Quarto document containing two Markdown grid tables, switch from Source to Visual and back, and inspect how the editor round-trips the document. Done means both tables and all values survive unchanged.