As per Section 2 (71) of the CGST Act, 2017, “location of the supplier of services” means,–
(a) where a supply is made from a place of business for which the registration has been obtained, the location of such place of business;
(b) where a supply is made from a place other than the place of business for which registration has been obtained (a fixed establishment elsewhere), the location of such fixed establishment;
(c) where a supply is made from more than one establishment, whether the place of business or fixed establishment, the location of the establishment most directly concerned with the provisions of the supply; and
(d) in absence of such places, the location of the usual place of residence of the supplier;
While supplier's action - as per facts narrated in your query - is odd, same is NOT necessarily mean that it is wrong / illegal. One does not know how supplier has structured its business operations and GST compliance strategy at his end.
Your query raised in from point of view of recipient of subject services who presumably wants to take ITC against IGST so charged by the supplier and concerned that such ITC will be disputed by Dept. in future. If so, my views are as under:
A. Taking ITC of IGST so charged can be legally defended judicially.
B. Kindly ensure that as per your written agreement & understanding with the supplier that in case of dispute raised in future by Dept., you will NOT allow the supplier to change the invoices raised & it is responsibility of the supplier (& not yours) to defend type of taxes charged by him and GSTN used to levy such taxes.
For above suggestion, it is presumed that address of supplier mentioned in your lease / rental agreement is that of Maharashtra & not that of Gujrat. Of-course, address of property given on lease / rental (which is different from address of supplier) will be that of Gujrat. BUT, if address of supplier mentioned in your lease / rental agreement is that of Gujrat, to avoid needless disputes etc., you should ideally ask him to levy CGST & SGST from supplier's gst registration taken in Gujrat.
These are ex facie views of mine and the same should not be construed as professional advice / suggestion.