Archiving all tasks in a folded project concatenates the next project to the first

If I start with:
```
Project 1:
  ✔ Completed Item
Project 2:
  ☐ 2's task
```
And fold Project 1 to get this:
```
Project 1: [...]
Project 2:
  ☐ 2's task
```
Archiving will erroneously produce this:
```
Project 1: [...]Project 2:
  ☐ 2's task
```
When it should be:
```
Project 1:
Project 2:
  ☐ 2's task
```