[FTS Collections] Index build issues with multi-collection index

Build : 7.0.0-2302

Setup :
3 node cluster = kv+fts , kv+fts+n1ql , kv+fts+n1ql

Steps to reproduce =
1. Create a bucket 'bucket.com'
2. Create 2 scopes in this bucket = scope1 and scope2
3. For scope1, create 2 collections = s1c1 and s1c2
4. For scope2, create 3 collections = s2c1, s2c2, s3c3
5. Load 10000 docs in each of these collections
6. Create FTS index s2c1idx on collection scope2.s2c1. Ensure that the index has 10000 docs indexed
7. Edit the FTS index s2c1idx to also index scope2.s2c2. Index should have 20000 docs now
8. Create an empty collection s2c5 on scope2
9. Edit the FTS index s2c1idx to also index scope2.s2c5. Index should be rebuilt

Expectation : Index should be rebuilt and should have 20000 docs.
Actual : Index building is stuck at 37.62% with 16960 docs