First, measurement equipment is not necessarily the problem, method could be to some degree, that is not how we test for TCR or any other PWW manufacturer does it. Secondly, you are trying to compare apples and oranges, film/foil and PWW resistors are not the same by any definition except that they are both resistors. You cannot compare a Vishay against a PWW for thermal behavior. I am not having any problems with the epoxy I have been using for months now (and it is not epoxy glue, not the same thing). I have not received any reports from any of my customers around the world as to any problems with my resistors. I have not seen or heard of any problems with resistors outside of their TCR specs, they are all close to the TCR of the wire used.
I do not know why you are seeing higher than normal TCRs but I see in your charts that neither mine nor the Fluke resistors are achieving a stable TCR plateau, while it more or less follows the ramp, you cannot tell if the internal temperature of the resistors have actually matched the chamber temperature. While you are seeing a indication of TCR linearity during the ramp, there is nothing to indicate thermal equilibrium. Once a resistor achieves thermal equilibrium, the measurement should flatten until the external temperature changes again, then after a time lag, the resistance should start to follow the temperature. In your charts I do not see any indications of thermal equilibrium in any of the resistor plots, I believe MisterDiodes also pointed this out.
The question of stress, built up by whatever factors, is complicated. Since the wire wound resistor inherently has some stress built into it simply by the winding process, ignoring any other source of stress, this 'barrel' stress takes some time to relieve. Since the 'stress' on the wire during winding is a variable that cannot be completely controlled, it cannot be stated with certainty how long it can take to relieve the bulk of it. Military grade resistors and standards are put through a fairly long process to relieve those stresses to a high degree.
Throw in whatever other external sources of stress may be present and things get more complicated, in many cases, some of the outside stresses are rather small and temporary, others can present more of a problem, but in all, these stresses generally add up to very small perturbations in the resistance.
In case you were wondering, those specs about 'long' term drift, so many PPM per year, that isn't a linear spec, it doesn't mean that a resistor will linearly drift from the initial value to 35 or 50 PPM or whatever at the end of the year. It means that the resistor can show changes within that limit at any time during the year, it might jump 35 PPM within days or even at a later time during the year, it can go up or down or both. This applies to all resistors, a spec for change only defines the end limits, it doesn't define how or when the change can occur. In most cases, but not all by any means, the lion's share of change will usually occur in the youth of the resistor, just what that will be depends on the individual resistor.
The same spools of wire, the same bobbins and shells are still being used, the only changes that have changed over time is the sealant and that has been improved as time has passed. The faulty epoxy sealant happened over two years ago and only affected two customers, it is time to stop fussing over that. The current sealant has been working quite well and there have been no complaints of resistors being out of spec for a long time. The epoxies used in end sealing are room temperature curing, they do not require elevated temperatures.
If you believe you have a resistor that is out of spec, I suggest you send it to me for evaluation and if found to be out of spec, it will be replaced.