Hi Alfonso,
Thanks for your patience. Here I describe what I did:
1. I delete all existed conn data and files.
2. I deleted the conn from MATLAB path.
3. I download conn again and covered with three patch files (conn_bidsdir.m, conn_importbids.m, and conn_process.m). I just found patch20200226.zip (conn_bidsdir.m, conn_importbids.m) from your relpy to me, and conn_process.m from your reply to maloman. And you update the conn_bidsdir.m in another relpy to me.
3. I set path in MATLAB again (reinstall). 
4. I also update spm12 using "spm_update update". 
5. I typed "which conn_importbids" and got:
D:\MATLAB\R2017b\toolbox\conn\conn_importbids.m
6. I typed "conn_checkdistributionfiles" got:
spm @ D:\MATLAB\R2017b\toolbox\spm12
conn @ D:\MATLAB\R2017b\toolbox\conn 
5. type conn in MATLAB command window to start conn.
6. I re-run the "Project.New (import..."
This issue is still there.
ps. I selected data from 2 subjects who shown 205 covariates when I import all 151 subjects, and re-run these 2 subjects separately. I noticed that this time their covariates back to 200 which is correct. So, this error only happens when I import all subjects. Hope this helpful. 

Thanks
Hongru