Question: Psi letter with indices

Hello,

In a mechanical model, I would like to use the letter Psi. However, when I use it, i have a small issue with some index which disappear.

Here the code I put at the beginning of my worksheet to be able to use the Psi letter

constants:= ({constants} minus {Psi})[]:
`evalf/Psi`:= proc() end proc:
`evalf/constant/Psi`:= proc() end proc:
unprotect(Psi);

With the following code,

ChgtVariables:={seq(psi[i](t)=Psi[i](t)-gamma0(t),i=1..4)};

I obtain the following result:

The index with Psi has disappear.

However, if i use the code:

ChgtVariables:={seq(psi[i](t)=PSI[i](t)-gamma0(t),i=1..4)};

I obtain the following result:

There is no problem of index with PSI.

Do you have a idea why the index has disappeared ? How can I do to use the Psi letter with a index ?

PS: the code associated attached here:

Psi.mw

Thank you for your help