Different materials, different jobs
The USG/CGC installation guide for its FIBEROCK AQUA-TOUGH tile backer tells installers to fill the 1/4-inch (6 mm) space between a tub and the bottom row of tile with flexible sealant, to caulk around soap dishes and other wall fixtures, and to fill vertical joints at inside corners and next to door jambs with flexible caulk rather than stiff cement grout. It is a Canadian product guide, so read it as an example of the grout-versus-caulk distinction, not as a universal specification for every shower; the instructions for the products actually installed in your bathroom are what apply.
Read the defect as a question, not a diagnosis
Caulk that cracks or pulls away from an edge has lost its adhesion or flexibility. Sherwin-Williams’ caulk-failure guide lists possible causes: lower-quality caulk, the wrong type of caulk for the job (for example, latex or vinyl caulk where there is prolonged contact with water or considerable movement), a surface that was not properly prepared or primed, and a bead applied too thin. These are possibilities to check, not findings about your bathroom.
Cracked grout at an inside corner or along the tub raises a different question: did that joint call for flexible caulk instead of grout? Because the USG/CGC guide assigns those locations to flexible material, regrouting the same joint in a matching color does not settle whether the right material is in place.
The visible finish is not the whole wet-area assembly
Grout and caulk are only the visible surface. The PNNL Building America Solution Center guide to walls behind showers and tubs states that cement board is not waterproof: it must be coated with a fluid-applied waterproofing, or a water-resistive barrier that allows drainage must be applied behind it. Water management in a tiled wet area depends on layers you cannot see.
That limits what the surface can tell you. Intact grout or a fresh bead of caulk cannot confirm that the hidden waterproofing is sound, and a damaged joint alone does not show how much of the wall needs repair. A surface repair and an assessment of concealed moisture answer different questions.
Note what you see without opening the wall
Write down where the defect is (between tiles, at a corner, or beside a fixture or the tub), when you first noticed it, whether an earlier repair has reopened, and any nearby staining or musty odor. Wide shots plus close-ups help you describe the pattern, although photos alone cannot establish the cause. Our guide to photos that help with a repair estimate covers what to capture.
Avoid removing tiles or opening a wall yourself to look for hidden mold. The EPA’s guide to mold and moisture says investigating hidden mold requires caution when it disturbs possible growth sites, because that can release a large amount of spores, and suggests considering an experienced professional if you believe you have a hidden mold problem.
Ask what a proposed repair actually addresses
When you talk with a qualified tile or waterproofing professional, ask: Which material belongs at this joint? What supports the proposed explanation? Does the work cover only the finish, or also the water-management layers behind it? What is still unverified? The answers help separate a cosmetic repair from a claim that the wet area is sound.
If there are signs of leaking or moisture damage, ask for the source to be found rather than treating fresh caulk as proof the problem is solved; the EPA guide stresses fixing the water problem, not only cleaning up after it. For related bathroom-moisture reading, see why a bathroom fan can run without clearing condensation. More questions are on our leak and water-damage repairs page, and local context is on the Northeast Philadelphia area page.