bug(gatsby-source-strapi): fetching content twice (via relationships) fails to transform to Markdown

## Describe the bug

It used to work TM.
One of my websites is failing to parse specific content to Markdown. I narrowed it down to happen when this entry is selected in a relation in a DZ component (that might matter). When I do not select the entry in that manyToMany relationship, the text gets converted to Markdown again.

## To Reproduce

Steps to reproduce the behavior:

1. Have a Questions model
2. Have a FAQ component with a manyToMany to the Questions
3. Have a DZ with this FAQ Component
4. See error

The Node should contain
``` content: { medias: [], data___NODE: '6a0df7ef-2907-52e4-80dd-147f931ace96' },```
But contains:
``` content: {
data: "My content",
medias: []
},
```

## Expected behavior

Rich text should always be available in Markdown

## System Info

System:
OS: macOS 12.6
CPU: (8) arm64 Apple M2
Shell: 5.8.1 - /bin/zsh
Binaries:
Node: 18.15.0 - /var/folders/50/3fndm5d13gs_kr96nwxpxd9c0000gn/T/yarn--1679606202261-0.9668410895557038/node
Yarn: 1.22.19 - /var/folders/50/3fndm5d13gs_kr96nwxpxd9c0000gn/T/yarn--1679606202261-0.9668410895557038/yarn
npm: 9.5.0 - /usr/local/bin/npm
Languages:
Python: 3.10.9 - /usr/local/bin/python
Browsers:
Chrome: 111.0.5563.110
Safari: 16.0

Start by reproducing the issue with the Questions model, FAQ manyToMany relationship, and DZ component described in the report. Trace the Gatsby source plugin's relationship fetching and rich-text-to-Markdown transformation, comparing output when the related entry is selected or omitted. Done means related content remains available in Markdown and the node has the expected data___NODE shape.

Written by the indexing model from the issue text.