Are you saying that your LR defaults are set to use the 28-75 lens profile no matter what lens you have, or that when you choose Auto or perhaps Default in the Lens Corrections area LR picks the wrong lens?
I always shoot with the 18-240mm Tamron lens. The metadata are always correct. However, when I move to the Develop module, and check Enable Profile Corrections, sometimes it shows the 18-270 lens, but other times it shows the 28-75mm lens, no matter whether I choose default or Auto - and it won't let me change it.
That seems to be a problem with the the Lens ID (happens on many Tamron lenses), to get it applied correctly each and every time you will need to create a develop preset and apply it during import. The preset (for Canon) would need to contain the following so check it after you copy the correct lens profile settings and create a new preset:
EnableLensCorrections = true,
LensManualDistortionAmount = 0,
LensProfileChromaticAberrationScale = 100,
LensProfileDigest = "A148EFF7414D8B6048E1F6F111BADAF5",
LensProfileDistortionScale = 100,
LensProfileEnable = 1,
LensProfileFilename = "Canon (TAMRON 18-270mm F3.5-6.3 DiII VC B003E) - RAW.lcp",
LensProfileName = "Adobe (TAMRON 18-270mm F/3.5-6.3 DiII VC B003E, Canon)",
LensProfileSetup = "Custom",
LensProfileVignettingScale = 100,
PerspectiveHorizontal = 0,
PerspectiveRotate = 0,
PerspectiveScale = 100,
PerspectiveVertical = 0,
ProcessVersion = "6.7",
Are you shooting RAW or JPG or some of both?
When you say it won't let you change it, does that mean there are no lens profiles at all, or no lens profiles that match your lens? If you set Auto or Default you can select any lens you want from the list of manufacturers and profiles and then the Setup says Custom. Of course this lens may not be the correct lens, but saying you cannot change it suggests there are no manufacturers or lenses listed on the dropdowns.
Most lens profiles are for RAW only, so if you shoot a mixture of RAW and JPG then for the JPGs the lens profile may not exist. If you are enabling Lens Profiles and have Auto or Default set and it chooses the wrong one then it may just be choosing the top one in the list for Canon if the specific profile doesn't exist for JPGs.
If you are shooting raw and the profile does exist then you should be able to correct the situation: In the list select the correct profile, then choose Setup: Save New Lens Profile Defaults, and select Default from the Setup instead of Auto and it should pick the correct lens every time. Assuming it does do that, then you can set Enable Lens Profiles = checked and Setup: Default and set those as your new Lightroom Defaults and it should pick the right lens.
If you are shooting JPG then the lens profile may well not exist and there's nothing you can do about that except not shoot JPG. If you feel like it you can hack a raw profile to be a jpg profile and put that hacked copy in the user-lens-profile area and LR will see it.