Does data anonymization conflict with GDPR rules?
Formally speaking, this is clarified in the GDPR Recital 26, unofficially titled Not Applicable to Anonymous Data:


The principles of data protection should therefore not apply to anonymous information, namely information which does not relate to an identified or identifiable natural person or to personal data rendered anonymous in such a manner that the data subject is not or no longer identifiable.
This Regulation does not therefore concern the processing of such anonymous information, including for statistical or research purposes.


Informally speaking, the claim that data anonymization would violate the data subject's rights to data erasure and portability, hence we should seek to use reversible anonymization techniques, sounds awkward, and against the very spirit of GDPR; and the official interpretation regarding anonymization is very clear:

Effective data anonymization is made up of two parts:

It is irreversible.
It is done in such a way that it is impossible (or extremely impractical) to identify the data subject.


In other words:

if a subject's data have been effectively anonymized, they are no longer personal data, hence they are no longer governed by GDPR; consequently, Articles 17 &amp; 20 are not applicable, and this does not constitute any conflict
if any personal data used as a source for the anonymized ones remain, they are subject to GDPR; data subjects can exercise their right to erasure and portability for these non-anonymized data
if the personal data used as a source for the anonymized ones are already erased (possibly in compliance to GDPR), then neither the right to erasure nor the right to portability are applicable any more, and this does not constitute any kind of conflict either.

Notice that the fact that ensuring effective anonymization is not as clear-cut as it may sound has also been legally recognized in the Opinion 05/2014 on Anonymisation Techniques of the Data Protection Working Party:

Thus, anonymisation should not be regarded as a one-off exercise and the attending risks should be reassessed regularly by data controllers.