StackingEstimator if template is used

Is it the following behaviour expected? I start tpot with a fixed template "Selector-Transformer-Regressor" but then I get a StackingEstimator model. I am not completely sure, but I think I never experienced that behaviour before the latest tpot update.

The final pipeline is:

```
[('rfe', RFE(estimator=LassoCV(alphas=None, copy_X=True, cv=5, eps=0.01,
fit_intercept=True, max_iter=1121, n_alphas=100,
n_jobs=None, normalize=True, positive=False,
precompute='auto', random_state=None, selection='cyclic',
tol=0.01, verbose=False),
n_features_to_select=6, step=0.11000000000000001, verbose=0)), ('stackingestimator', StackingEstimator(estimator=Earth(allow_linear=None, allow_missing=False,
check_every=None, enable_pruning=True,
endspan=None, endspan_alpha=None, fast_K=None,
fast_h=None, feature_importance_type=None,
max_degree=1, max_terms=None,
min_search_points=None, minspan=None,
minspan_alpha=None, penalty=None, smooth=None,
thresh=None, use_fast=None, verbose=0,
zero_tol=None))), ('elasticnetcv', ElasticNetCV(alphas=None, copy_X=True, cv=4, eps=0.1, fit_intercept=False,
l1_ratio=0.73, max_iter=1232, n_alphas=100, n_jobs=None,
normalize=True, positive=False, precompute='auto',
random_state=None, selection='random', tol=0.01, verbose=0))]
```