This problem is addressed in PRISMA 2009, PRISMA 2020 and MECIR. Subgroup analyses should be pre-specified, kept to a minimum and represent potentially important clinical differences. Unplanned or unjustified combinations of different trials in meta-analyses represent a way of 'massaging the data'. This can also be thought of as 'p-hacking'.
Articles that support this problem:
Can Cochrane Reviews in controversial areas be biased? A sensitivity analysis based on the protocol of a Systematic Cochrane Review on low-level laser therapy in osteoarthritis
2005 : Photomedicine and laser therapy
Many scenarios exist for selective inclusion and reporting of results in randomized trials and systematic reviews
2013 : Journal of clinical epidemiology
Attention should be given to multiplicity issues in systematic reviews
2008 : Journal of clinical epidemiology
Exploring treatment by covariate interactions using subgroup analysis and meta-regression in cochrane reviews: a review of recent practice
2015 : Plos one
Statistical multiplicity in systematic reviews of anaesthesia interventions: a quantification and comparison between Cochrane and non-Cochrane reviews
2011 : Plos one
Reporting quality of Cochrane systematic reviews with Chinese herbal medicines
2019 : Systematic reviews
Lessons learnt on transparency, scientific process and publication ethics. The short story of a long journey to get into the public domain unpublished data, methodological flaws and bias of the Cochrane HPV vaccines review
2019 : Bmj evidence-based medicine
Age-treatment subgroup analyses in Cochrane intervention reviews: a meta-epidemiological study
2019 : Bmc medicine
Flaws in the application and interpretation of statistical analyses in systematic reviews of therapeutic interventions were common: a cross-sectional analysis
2018 : Journal of clinical epidemiology
Statistical heterogeneity in systematic reviews of clinical trials: a critical appraisal of guidelines and practice
2002 : Journal of health services research & policy
The Reporting Quality of Systematic Reviews in Japanese Physical Therapy Journals
2020 : Progress in rehabilitation medicine
The collaboration and reporting quality of social welfare systematic reviews in the Campbell Collaboration online library
2019 : Health & quality of life outcomes
The fate of urological systematic reviews registered in PROSPERO
2019 : World journal of urology
Is quality control of Cochrane reviews in controversial areas sufficient?
2006 : Journal of alternative & complementary medicine
Caution should be exercised when assessing ivermectin for the treatment of COVID-19 in systematic reviews
2022 : Reviews in medical virology
Saturated fat, the estimated absolute risk and certainty of risk for mortality and major cancer and cardiometabolic outcomes: an overview of systematic reviews
2023 : Systematic reviews